This proposal concerns the search for improved scintillator crystals for positron emission tomography (PET) arrays. It is based on an earlier discovery, made jointly with the Lawrence Berkeley Laboratory, that PbSO4 crystals, although very difficult to grow, have better PET scintillation properties than any other material previously studied. The Phase I goals were (A) the search for a scintillator crystal, equivalent to or better than PbSO4, that is economical to grow and (B)the search for an economical technique for growing PbSO crystals. Our Phase l activities resulted in (A1) the screening of several thousand compounds to yield 51 candidates that have a high probability of including any easily grown scintillators, (A2) the procurement/synthesis and testing of 18 candidates for scintillation evaluation, and (B) the Identification of a process for growing clear scintillating PbSO4 crystals. The Phase II goals are (l) formulation of a figure of merit for evaluating PET scintillators, (2) evaluation of three potentially low-cost techniques for growing PbSO4 crystals, (3) continued evaluation of alternatives to PbSO4, (4) scintillation testing and evaluation of crystals, (5) comparative cost analysis of crystal growth techniques and (6) optimization of crystal growth and fabrication of demonstration scintillators. The Phase III objective will be the commercial growth and distribution of optimal PET scintillator crystals for medical research and diagnosis systems. PROPOSED COMMERCIAL APPLICATION: Production and sale of scintillator crystals at the ultimate estimated rate of about 1-2 metric tons per year at a price of $2,000,000 to $5,000,000 per ton.